Heslin, a 50-year-old construction worker, says he normally pays little attention to politics. But he was yanked painfully into the middle of the nation’s gun debate last December, when his 6-year-old son, Jesse, along with 19 other first-graders and six educators, was shot dead at Sandy Hook Elementary School in Newtown, Conn.

“It’s a burden, it’s more than a burden on me,” Heslin said in an interview Tuesday as he and three dozen others — including other Newtown families and relatives of other mass shooting victims — arrived in Washington for two days of lobbying lawmakers. “But I have to do it for my little boy.”

Heslin is set to testify to the Senate Judiciary Committee on Wednesday in support of legislation by Sen. Dianne Feinstein, D-Calif., to ban assault weapons.

Other witnesses testifying to the Senate panel include William Begg, an emergency room doctor who treated Newtown victims that day, and U.S. attorney John Walsh from Colorado.

“Guns that are fashioned from war don’t belong on the streets,” Feinstein said Tuesday, acknowledging that her legislation to ban assault weapons faced difficult odds in Congress. “Maybe I’ve just seen too much from my days as mayor and watching this stuff for 30 years.”

Feinstein, who rose to become San Francisco mayor, was on the city’s board of supervisors in 1978 when Mayor George Moscone and supervisor Harvey Milk were fatally shot in City Hall.

Across the Capitol on Wednesday, the House Education and Workforce Committee planned to hear from school safety experts and counselors about how to keep students safe.

Witnesses testifying to the Republican-controlled House panel were expected to emphasize the role of school resource officers — security professionals who are often armed and can double as informal counselors and liaisons to law enforcement. Those officers are commonplace in many schools and help officials develop safety plans.

Mo Canady, executive director of the National Association of School Resource Officers, was among those slated to testify, along with a school counselor and a school safety director.

Heslin and his group met with around six lawmakers and aides Tuesday, mostly Senate Democrats from Republican-leaning states. Participants said House Majority Leader Eric Cantor, R-Va., said he would try to help and Senate Majority Leader Harry Reid, D-Nev., expressed optimism that the Senate would produce gun legislation, but neither committed to anything specific.

In his prepared Senate testimony, Heslin said he’s been told his son died yelling to people to run. He said Jesse was hit by one bullet grazing the side of his head, another hitting his forehead.

“That means the last thing my son did was look Adam Lanza straight in the face and scream to his classmates to run,” Heslin said, referring to the 20-year-old who committed the massacre. “The last thing he saw was that coward’s eyes.”

Despite the raw emotion, Feinstein’s effort to ban assault weapons is expected to fall short due to opposition by the National Rifle Association and many Republicans, plus wariness by moderate Democrats.

Feinstein’s bill has attracted 21 co-sponsors, all Democrats. Including herself, it is sponsored by eight of the 10 Judiciary panel Democrats — precarious for a committee where Democrats outnumber Republicans 10-8. Democrats on the panel who haven’t co-sponsored the measure include the chairman, Pat Leahy of Vermont, who said Monday he hadn’t seen the bill.

President Barack Obama made bans on assault weapons and large capacity magazines key parts of the gun curbs he proposed in January in response to the Connecticut school massacre.

The cornerstone of his package is a call for universal background checks for gun buyers, some version of which seems to have a stronger chance of moving through Congress. Currently, only sales by federally licensed gun dealers require such checks, which are designed to prevent criminals and others from obtaining firearms.

Obama also proposed providing more money to school districts to hire school resource officers and counselors and take other safety steps.

Feinstein’s bill would ban future sales of assault weapons and magazines carrying more than 10 rounds of ammunition but exempt those that already exist. It would bar sales, manufacturing and imports of semiautomatic rifles and pistols that can use detachable magazines and have threaded barrels or other military features. The measure specifically bans 157 firearms but excludes 2,258 others in an effort to avoid barring hunting and sporting weapons.

Feinstein, who helped create a 1994 assault weapons ban that expired in 2004, and other supporters cite studies showing use of the firearms in crimes diminished while the prohibition lasted. A 2004 report said the proportion of gun crimes involving assault weapons dropped by up to 72 percent in five cities studied.

Opponents cite data from that same study showing assault weapons were used in only 2 percent to 8 percent of gun crimes, arguing that a ban would have little impact. The study also estimated there were 1.5 million assault weapons owned privately in the U.S. in 1994, and an estimated 30 million high-capacity magazines as of 1999, which critics say means exempting them would diminish a ban’s effect.